Frequency modulation



Ami 3a; 1946. 5. L. USSEtMAN 2,39%,26

v FREQUENCY" MODULATION I Filed July 20, 1943 v 4 Sheets-Sheet 1 WJJJJJJJA X INVEN TOR.

1 April 30, 1946. G. L USSELMAN FREQUENCY MODULATION Filed July 20, 1943 4 heets-Sheet 2 R W n m M 2 m N. f i p a Y B q mx April G. L. USSELMAN 2,399,263

FREQUENCY MODULATION Filed July 20, 1943 4 Sheets-Shem; 3

INVENTOR.

Patente Apr. 30,1946

2,339,268 maeusncr MonUmrroN Application July 20, 1943, Serial No. 495,459 7 Glaims. (on. lie-171.5)

This application discloses an improved means for producing wave energy the timing of which is modulated in accordance with signals.

In the improved system of the present.appl.ication, I make use of the principle involved in my U. S. application Serial No. 338,837, filed June 5, 1940, now U. S. Patent #2,298,436; U. S. applicationSerial No. 338,838, filed June 5, 1940, now U. S. Patent #2,298,43'7'; and U. S. application Serial No. 361,506, filed October 17, 1940, now U. S. Patent it 2,298,438.

In systems of this type the oscillations to be timing modulated are developed in a first electron discharge device, are fed by coupling means therefrom to the control electrode of a second electron discharge device which may be termed the modulator tube, are amplified in the second device and fedfrom the anode by coupling means back to the generator circuit. A phase shifting means is provided in one of the couplings so that the voltages fed back to the generator are substantially in phase quadrature with the gen-'- erated voltage. This in quadrature voltage combines with the generated voltage to produce a resultant of a predetermined phase with respect to the generated voltage in the absence of modulation. When the component fed back is amplitude modulated by controlling the second tube the phase of this resultant varies to vary the phase of excitation of an electrode in the generator and as a consequence, of the voltage fed back to the modulator tube and returned to the generator, so that the efiect is cumulative and the timing of the oscillations generated is modulated.

In the modulators of the prior applications, crystal controlled oscillators are used and certain of the oscillators, except for the crystal,.are aperiodic. The coupling circuits between the first and secondtubes, however, are tuned, this limiting the range of operation. By the term tuned circuit or frequency determining circuit applicant means a circuit resonant at some frequency at which oscillation can start and/or be maintained.

An object of the present invention is to improve systems of the type disclosed in said prior applications.

A more specific object of the present invention is to provide in a system of this type an oscillator that, except for the oscillator frequency determining circuit, which may be a crystal, is aperiodic or untuned, and a modulator tube having its grid coupled for excitation to the generator by a coupling. which is untuned and having '"cuit that does not use tuned reactances.

its anode coupled back to the generator by a gige phase shifting means may be in either of the couplings between the modulator tube and the oscillation generator. In a preferred embodiment it is in the coupling between the output electrodes of the modulator tube and, the oscillation generating circuit. The phase shifter, per se, may comprise resistance and inductance or resistance and capacity and this circuit is also aperiodic or untuned. The system of the present invention then is substantially free of frequency determining circuits except for the crystal or its equivalent.

Numerous advantages are obtained by the use of my improved system. For example, by the use of untuned circuits and resistive impedances in the network, the voltages fed back by the v phase shifting networks is substantially constant in value regardless of the operating frequency, assuming that the phase shifting reactors are correctly adjusted for each oscillator frequency. This is a desirable feature, wherein it is intended that the modulator be operable at a number of different frequencies.

Since there is only one frequency determining element in the entire circuit, i. e., the crystal or a parallel tuned circuit serving the function of the crystal, to change the-operating range requires only that a crystal of different frequency be substituted, or the parallel tuned circuit tuning be changed, and adjustment of the phase shifting reactors,'which is not critical, be made.

In the various modifications illustrated, the output generator is coupled electronically to the generator circuit and electrodes and, as a consequence, this tuned circuitis not in the timing modulation system and has little effect on it. Of course-when the frequency of operation of the generator and modulator is changed, a. cor- Another advantage obtained by the use of my 7 improved system, is that since the crystal or the paarllel tuned circuit used in place of the same is the only frequency determining element inthe system wherein timing modulation is accomplished, the system is very stable in operation.

- Moreover, since the system-uses substantially entirely resistance coupling or resistance and capacity or resistance and inductance couplings or choking impedance coupling, it may be built economically and occupies a comparatively small space. 1

The sy tem is adapted to timing modulation of wave energy in accordance with signals such as voice, etc., and also to timing modulation in accordance with telegraphy signals, in which case the output is keyed spaced waves, i. e comprises a carrier and side bands resulting from keying which carrier shifts from a first frequency which may be called the spacing frequency to a second frequency which may be called the marking frequency.

Numerous other objects of my invention, the manner in which they are attained, and the advantages flowing therefrom, will appear from the detailed description which follows, and in which reference is made to the attached drawings. In the drawings Figs. 1, 3, 3a, 5, and 50. each show an embodiment of my improved wave generating and waving timing modulating system.

Figs. 2 and 4 are vector diagrams of voltages which appear on the electrodes of the systems of Figs. 1 and 3, respectively, and these vectors are used in explaining the operation of the invention.

Fig. 1a is a modification of the arrangement of -Fig. 1, while Fig. 3a is a modification of the arrangement of Fig. 3.

In Fig. 1 of the drawings, tube VI, which is a generator tube, has an anode II) connected to a tank circuit comprising condenser CI and inductance Ll from which the timing modulation oscillations are induced into an inductance L2 and supplied to additional frequency multipliers. am-

plitude limiters, etc., before use or more directlyto radiating means. The inductance LI also serves as a means for supplying direct current potential to the anode III. The direct current source .(not shown) is shunted by a radiofrenuency'bypass condenser connecting one end of LI to ground. The tube V! has an electrode I2, a control grid I4, and a cathode K coupled in an oscillation generating circuit including piezoelectric crystal X. This oscillator circuit is of the grounded anode type, wherein electrode I2 serves' as the anode and is grounded by radio frequency bypass condenser O.

grid I4 and anode I2, so that the oscillating voltages on the said two electrodes are of opposed phase, while the cathode is tapped up on resistance RI and the voltage thereat is intermediate. the voltages on the electrodes I2 and I4. The operation of this oscillator wherein oscillations are generated and fed by the electron stream to the circuit CI, LI has been described in the patents mentioned above and is well known in the art. Further description thereof is believed unnecessary here.

The control grid 24 of tube V2 which is the modulator tube is coupled by condenser 26 to a point on the resistor RI, so that voltage ofthe generated frequency is fed to the control grid 2. This voltage is amplified in tube V2 reversed in phase and fed from the anode 28 thereof by way of a phase shifting network comprising condensers C2, C2 and resistorsRS and R4 to a plate II of the crystal .X. The control grid 24 is also coupled by gridresistor-Ri to the secondary winding of a transformer TI, the primary winding" of which is coupled to a signal source A. The transformer winding end of resistance R! is coupled to the cathode by a radio frequency bypassing and modulation potential blocking condenser O". A grid bias source 22, shunted by a modulation potential coupling condenser O, is in the circuit of the secondary winding of transformer T. The anode of V2 derives its potential 6 quency by a condenser O. A radio frequency bypass condenser O is also connected between the lower end of R6 and ground and the cathode of tube V2.

The condensers labeled 0 in the drawings are 10 radio frequency bypassing and/or direct current blocking condensers. The condenser 0' is of suf- I ficient size to act as a modulation frequency coupling condenser. Also condenser O" is sufficiently large to act as a bypass for the carrier irequency but it is a high impedance for the modulating frequencies.

The operation of the modification illustrated in Fig. 1 will now be described. As stated above. tube VI operates in a grounded anode type of oscillator circuit. It will be seen that the crystal X is the only frequency determining element in this generator. The output circuit CI, LI, L2 is electronically coupled to the oscillation generatin electrodes and circuits.

Frequency modulation is accomplished by means of amplifier tube V2, which operates by anode impedance R8 or L3 through a phase shift-- ing network C2, R3 and C3, R4 to feed to the electrode SI of the crystal X a voltage which with so respect to the voltage exciting grid 24, is reversed in phase in tube V2 and shifted about 90. in phase in the phase shifting network.

The principle of operation-is as follows, assuming that the crystal oscillator is oscillatlngwith at proper potentials being applied to all circuits and electrodes. Referring to vector diagram Fig. 2 the generated radio frequency voltage on cathode K of tube VI is represented by vector VIK. This cathode potential is coupled to the control grid of tube VI through the interelectrode capacity and through resistor RI. A part of the voltage between cathode K and ground is supplied to the control grid 24 of tube V2 through the low impedance coupling condenser 26. This volta e is amplified in tube V2 and reversed 180 in phase at the anode of tube V2. This amplified voltage is shown in Fig. 2 by vector WA. The radio frequency voltage from the anode of tube V2 is passed through the phase shifting network C2,

R3 and C3, R4. In passing through the phase shifting network C2, R2 and C3, RA, the radio frequency voltage is advanced in phase say about 90 and reduced to about one-third or less in amplitude. Larger reactances produce a smaller 55 output and larger phase shift. Smaller reactances conversely give large output and smaller phase shift. This voltage is represented in Fig. 2 by vector XI This latter voltage is applied to the crystal K through the crystal electrode II. The

to radio frequency voltageof cathode K is coupled to the crystal by crystal electrode )f through a portion of resistor RI and through the grid to cathode capacity of tube VI. The grid Il thus has a generated voltage of .a first phase which is on s the crystal and the crystal gets an amplified phase,

quadrature voltage which is'amplitude modulated so the grid I4 gets a resultant voltage of shifting phase. It may be said that the excitation on the control grid of tube VI is represented by vector 7o .xI2- which is the vector sum of cathode excitaion vector VIK and the phase shifted excitation ector XI. i Thecontrol gridll or'tubc van also provided with modulating potentials from transformer Ti so that the amplitude ofthe anode current and heads.

the voltage represented by vector XI applied to the crystal electrode 3! is responsive to the signalling currents. Due to the shape of the tube characteristics the amplitude of the radio frequency oscillations in the output of tube V2 follows the amplitude and oscillations of the signal, In other words, the radio frequency excitation to and from asoaaee tube V2 rides on top of the modulation signal oscillations.

The amplitude of the radio frequency excitation applied through condenser 26 to the grid of tube- V2 is substantially constant but in the output of tube V2 the radio frequency excitation applied to the network C2R3, C3114 and thence to the crystal electrode 3! is amplified according to the signal oscillations. This radio frequency excitation, amplified according to the signal, is phase shifted and applied to the crystal as explained above. The radio frequency excitation represented by vector XI follows the amplitude of the signal oscillations so that vector XI grows longer and shorter, as shown in Fig. 2 by arrow substantially constant vector VlK results in a variable vector XIZ, which varies in phase as well as amplitude. This phase variation of Xl2 is fed from the crystal to the grid M and from the oscillation generator back through tube V2, accumulatively to result in a frequency or timing due to action of the rectified grid current and the crystal holder capacity. In this manner, the frequency of the crystal oscillator is modulated according to the signal oscillations.

It will be noted that the crystal X is the only frequency critical reactance inthe .system. The .phase shifting reactors are not critical to frequency but their 'impedances change gradually as the frequency changes. What is meant here is that the crystal is the only element that is frequency determining and that will start and/or maintain oscillations. The crystal X may be replaced by a parallel tuned circuit where larger deviations than are obtainable by use of the crystal are desired. The amount of deviation of course depends to a large extent on the Q of the parallel tuned circuit, the frequency swing being reduced as the Q of the circuit is increased.

In the modification in Fig. 1, the source A may represent voice or similar signals. Where telegraphy or code signals are to be used to produce spaced wave keying, the arrangementof Fig. 1 may be modified as indicated in Fig, 1a. This is accomplished by breaking the circuits at the points marked with crosses in Fig. 1 to remove the tube V2 and the modulator circuits, including transformer Ti and source A, and replace the same by the keying circuit of Fig. la. y

When Fig. 1 is modified as illustrated in Fig. 1a; closing of the key 21 applies to the control grid 24, a potential from source'23 which varies between a negative value and a higher negative value to thereby vary the amplitude of the phase shifted voltage supplied by tube V2 to the crystal electrode 8| represented by vector XI in Fig. 2, from a first amplitude to a second amplitude to shift the phase of the resultant voltage on the grid Id of tube VI from a firstphase to a second phase so that the frequency of the generatedoscillations is shifted as the system is keyed from a first frequency, which may be designated the spacing frequency, to a second frequency, which anode. The addition of variable vector XI and .may be designated the marking frequency. The system operates in other respects substantially as described above in detail in connection with Fig. 1.

In Fig. 5, I have shown a preferred embodiment of my system. This figure is in many respects similar to the modifications of Fig. 1 and Fig. 1a. In the arrangement of Fig. 5, however, a switch 50 is provided to permit timing modulation of the generated oscillations in ac-' cordance with varying energy such as voice sig nals supplied by transformer T when switch 50 is to the right, or in accordance with code or tele raphy systems produced by keying the potential on the grid 2% when the switch 50 is in the left hand position.

Moreover, in this system, a crystal X having but two electrodes is used. This crystal is in an oscillation generating circuit of the grounded anode type substantially .the same as the oscillator of Fig. l. The grid I2 is the oscillator Grid I3 is similar to the anode in that it operates at positive potential and when it is not modulated (by amplified R. F. from tube V2) it may aid the anode grid l2 in maintaining oscillations. Note that if R6 is reduced to approach zero value grid I3 being connected to ground by an R. F. bypass condenser 0 would act like an oscillator anode as much as grid l2. However, since some impedance is placed between grid I 3 and ground it becomes a controlling grid sented by the vector system of Fig. 2. Here as in Fig. 1 the radio frequency fed back by the tube V2 and phase shifting network C2R3 and C3R4 is about in quadrature with the generated voltage on the grid M and is modulated in amplitude. The generated voltage and the amplifled and returned voltage both control the electron stream which causes the radio frequency voltage on the cathode K. Modulation of the voltage returned to the grid l3 modulates the phase of the oscillatory energy developed. This .modulation of the phase of the oscillatory energy developed and fed back through condenser 26 to electrode 24 is accumulative and timing modulation of the oscillatory energy generated is accomplished substantially as described in detail in connection with Fig. 1.

Moreover, in this modification the condensers C2 and C3 ar made variable so that the frequency of operation may be quickly and easily changed merely by adjusting the condensers C2 and C3 and changing the crystal X by switching in a second crystal or replacing the one shown.

This permaincrystal in the oscillator generating circuit to place thereon a damping effect. If a low impedance network were used in Fig. 1, it might reduce the ability of the oscillation generator. to produce oscillatory energy of good amplitude continuously. In Fig. the impedance network is terminated by the impedance between the electrode l3 and cathode K. As a matter of fact, it is advantageous to use a low impedance modulator tube V2 and to make the phaseshifting network impedance match the output impedance of tube V2. In this modification note that the grid I3 is supplied by direct current through phase shifting resistance R4 which in this respect has a double purpose. a

The modification illustrated in Fig. 5a differs from the modification illustrated in Fig. 5 in the following .main respects. In Fig. 5a the output is coupled between ground and the anode [0 of the tube VI, so that the potential drop produced across resistor I I is fed to the output. The sup-- ply for the-anode 28 of tube V2 is by way of resistance R6, L3 being omitted, so that here we have again a system where there are no tuned circuits or frequency determining elements except the crystal which is in the oscillation circuit. This modification is very stable in operation due to the absence of tuning reactances, etc., and

' may be built in a comparatively small space. If

th same is used as an exciter for frequency vmultiplying stages the desired frequency swing is readily obtainable and the system is well adapted for frequency shift keying as described above.

In the modifications discussed above it will be understood that although I have shown phase advancing networks C2R3, 03R, I may use a phase retarding networks wherein inductances 1 replace C2 and C3 without departing from the spirit of the present invention.

The single tube modulator arrangements described above are well suited to frequency shift keying since the modulator tube anode current may be completely cut off for one part of the telegraphy signal, say, for example, on space.

Where a larger amount of swing is desired and where reduced amplitude variation is desired, I use a pushpull modulator arrangement as illustrated in Fig. 3. Such an arrangement is preferable for phone modulation since less distortion is produced due to the balancing action of the modulator. In this arrangement the oscillation generator and outputcircuit comprising tube Vi audits electrodes and their connections is substantially as illustrated in Fig. 1 and a description thereof is believed unnecessary. The modulation system, however, comprises two electron systems V2 and V2 which may be in a com-.

mon envelope or separate envelopes. The excitation voltages fed from resistance RI 'goes to a grid, 24 and 24, of each tube through coupling condensers 26 and 26'. v The tubes in this modification are difierentially modulated by potentials supplied from source A through transformer '1" and resistances R5 and R5 to grids 24 and 24'. The modulation circuits and bias circuits for the tube'systems V2 and V2 are similanta thecortromagnetically isolated from each other, either by spacing or by shielding.

In this modification then the excitation voltages fed back by electron system V2 is advanced and the voltage fed back by electron system V2 is retarded.

Fig. 4 showsthe vector relations of the radio frequency potentials in Fig. 3. In general, the voltage relation for the generator VI and each modulator V2 and V2 are similar to the relations represented in Fig. 2. It may be desirable to have more phase shift of the radio frequency potentials supplied to crystal electrodes 3| and 4|, as shown by vectors X3l and X of Fig. 4. This reduces the angle between KM and X and results in somewhat less frequency deviation but the stability is thereby improved. However, the total frequency deviation in Fig. 3 is much greater than that which may be obtained by the arrangements of the prior modifications because of the pushpull modulator action. In Fig. 3, I have the addition of three voltages on the grid l4 and these voltages are represented by three vectors, VIK, X3! and X41, as shown in Fig. 4. These voltages give the resultant voltage represented by the vector X51 on the grid l4 oftube V2. Vectors X3! and X are varied differentially by the signal'oscillations which causes vector XSI to vary or oscillate in phase but to remain, between the positions X5l' and X5l", substantially constant in amplitude. Vectors X5l" and X51 may be omitted if one assumes that vector X5I is the resultant of VIK,X3I and X.

A preferred embodiment of my improved system is shown in Fig. 3a. This modification has features in common with the modification of Fig. 3 and resembles in some respects the modifica tion of Fig. 5.

In Fig. 3a the oscillator tube VI has in addition to the oscillation generating electrodes l2, l4 and K, two coplanar grids l5 and I5 both oi which are at positive potential by virtue of their connections to the positive terminal of a source function somewhat similar to the grid l3 of Fig.

respondingcircuits of tubeVTin Fig. I, described in. detail hereinbefore.

The anode 28 of tubeVZ feeds through a phase advancingnetwork C2R3, 63R toan electrode 3| on the crystal, so that the system so far described operates substantially in the same manner in which the system of Fig. l operates; The anod 28' of the electron system V2 is coupled 5 as described above.

Two phase shifting networks C2R3, C334 and LSRI, L6R8 connect the anodes 2B and 28, respectively, of the pushpull modulator tube systems to the grid I5 and I5, respectively. This modification is similar to the modification. of Fig. 3 in that pushpull modulators are used and arranged substantially as illustrated in Fig. 3.

The two phase shifting condensers C2 and C3 have defimte impedance values of the same magnitude as that of the inductances L5 andLB in the other phase shiftingnetwork. By the use of;

iroirdusucore inductances the same can be made- "variable and compact and be of. small dimensions:

Thecondensers 0" between" RJ and R8 and ground are radio frequency and direct current blocking condensers.

The operationhere is substantially as described above in connection with 3, and as illustrated by the vector diagram of Fig. 4.

Due to the use of the pushpullmodulator in substantially eliminated. Except tor the diner ent features pointed out above the operation of the frequency modulator shown in Figs. 3 and 3a is in general similar to that of Figs. 1, and 5a. In the systems of Figs- 3 and 3a, one of the electron tube systems, say for example, V2, may

be removed. The operation will then be as in Figs. 1 and 5. Howeventhe voltage relations represented by the vectors in Fig. 2 will difier in that the voltage fed from the anode 28' will be retarded the desired amount by the action of inductances L5 and L5, instead of being advanced as is the case in Figs. 1 and 5, where the phase shifting networks include condensers C2 and C3. I

It is preferable, in some respects, at least, to use a pure resistance in the place of choke L3 to carry anode current to the modulator tube V2 in Figs. 1 and 5, and in place of chokes L3 and Lfii in Fig. 3. The choke coils are somewhat more (if? v ficient but a resistive impedance is desirable for this purpose, because it interferes less with the.

action of the phase shifting networks and because it is constant in value regardless of the operating frequency. The latter is desirable where it is intended that the modulator be tunable to a number of frequencies. v

It will be readily recognized by those skilled in the radio art, that the values of the circuit elements depend to a large extent on the frequency of the generated waves, and consequently, I do not wish to limit myself to elements of a particular value. However, in systems which operated satisfactorily circuit elements as follows were used. In Fig. 1 condensers 26 and O and O are all of comparatively large capacity and low radio 'frequency impedance, used for blocking direct current and bypassing radio frequency. Condensers 26, 26', C6 and O of Figs. 3 and 3a are also directcurrent blocking and radio frequency bypassing condensers. In Fig. 3 the choke coil impedances L3 and L3 are so located that they have no coupling to each other as are L5 and L8. The grid resistors RI of Figs. 1 and 3, are of the order of, 1 megohm. The phase shifting reoscillation generating circuit amplified voltages reactance, a pair of electron discharge tube systems each having a control electrode, a cathode and an anode, connections between the control electrodes and cathodes ofsaid additional el'ectron discharge tube systems and said oscillation generating circuits for impressing on saidcontrol electrodes excitation voltages of the generated frequency, a first phase shifting circuit including series reactance and. shunt resistance coupling the anode and cathode of one of said pair of electron discharge tube systems to said oscillation generating circuit, a second phaseshifting' circuit including series reactance and shunt resistance coupling the anode and cathode of the other of said pair of electron discharge tube systems to said oscillation generating circuit, said phase shifting circuits serving to feed to said of the generated frequency, the said frequency determining reactance being the only frequency determining reactance in said circuits, and connections for differentially controlling the gain of a ing electrode coupled in an pscil1ationgenerating circuit including as the sole frequency determining reactance a piezo-electric crystal in a holder having a plurality of terminals, a pair of electron discharge tube systems each having a control electrode, a cathode and an anode, connections between the control electrodes and cathodes of said additional electron tube systems and said oscillation generating circuit for impressing on sistors R3 and Rd of Figs. 1 and 3, and R1, R8 of 1 Fig. 3, are of the order of 25,000 to 50,000 ohms. However, if wider crystal gaps are used, that is less coupling, smaller resistances in the order of 5,000 to 10,000 ohms may be used. Condensers C2 and C3 of Fig. I may each be made to have the same impedance as R3 and R4. Inductance coils L5 and L6 and condensers C2 and C3 of Fig. 3 may each be made to have an impedance value equal to or somewhat more than the resistance of each resistor R7, R8, R3 or R0. These phase shifting resistors in any one circuit may generally have the same resistance. The same may be said for the phase shifting coils and condensers in regard to reactance. However, these impedance values may be changed in any desirable manner provided the desired phase'shiit is obtained and the impedance of the phase shifter network at the output of the modulator tube and I the oscillator is of the desired value.

- control electrode, and an electron stream producing electrode coupled in an oscillation generating circuit including a frequency determining said control electrodes excitation voltages of the generated frequency, a first phase shifting circuit including series inductance and shunt resistance coupling the anode and cathode of one of said pair of electron discharge tube systems to a pair of terminals of said crystal holder, a second phase shifting circuit including series capacity and shunt resistance coupling the anode and cathode of the other of said pair of electron discharge tube systems to a pair of terminals of said crystal ating circuit, said device having two additional electrodes, a pair of electron discharge tube systems each having a control electrode, a cathode and an anode, connections between the control electrode'and cathode of each of said additional electron discharge tube systems and said oscillation generator circuit for impressing on the control electrodes of said tube systems excitation voltages of the generated frequency, a first phase shifting circuit coupling the anode and cathode of one of said additional electron discharge tube systems to the cathode and one of said additional electrodes of said device, a second phase shifting circuit coupling the anode and cathode of the other of said additional electron discharge tube systems to the cathode and the other of said additional electrodes oi! said device and connections for difierentially controlling the gain of said additional electron discharge tube systems to correspondingly modulate the timing of-the oscillations generated. a

4. Apparatus as recited in claim 3, wherein said additional electrodesoi said device are co-planar.

5. In a wave length modulation system an oscillation generator including a tube having electrodes including ananode, a grid-like anode,

anode, a piano-electric crystal in a holder having two terminals, connections including said first grid-like anode, said control grid and said cathode in an oscillation generating circuit including said crystal in its holder between the said control grid and cathode of said tube, an additional electron discharge device having an anode and having a control electrode coupled to said generating circuit to derive excitation voltage therefrom, a phase shifting circuit comprising series reactance and shunt resistance coupling the anode and cathode 01 said second named device to the second grid-like anode and the cathode of said first tube, means for controlling the gain of said additional device in accordance with control potentials to modulate the wave length of the oscillations generated, and an output circuit coupled to the anode and cathode of the first tube.

6. In apparatus of the class described, an electron discharge device having electrodes including an electron receiving electrode, an electron flow control electrode, and an electron stream producing electrode coupled in an oscillation generating circuit including a frequency determining react? ance, a pair of electron discharge tube systems each having a control electrode, a cathode and an anode, connections between the control electrodes and cathodes of said additional electron discharge tube systems and said oscillation generating circuit for impressing on said control eleca cathode, a control grid and a second grid-like trodes excitation voltages of the generated frequency, a first phase shifting circuit coupling the anode and cathode of one of said pair of said electron discharge tube systems to said oscillation generating circuit, a second phase shifting circuit coupling the anode and cathode of the other of said'pairof electron discharge tube systems to said oscillation generating circuit, said phase shifting circuits serving to feed to said oscillation generating circuit amplified voltages of the generated frequency, and connections fordifierentially controlling the gain of said additional electron discharge tube systems to correspondingly modulate the timing of the oscilla holder having a plurality of terminals, a pair of electron discharge tube systems each having a control electrode, a cathode and an anode, connections between the control electrodes and cathodes of said additional electron discharge tube systems and said oscillation generating circuit for impressing on said control electrodes excitation voltagesof the generated frequency, a first phase shifting network coupling the anode and cathode of one of said pair of electron discharge tube systems to a pair of terminals of said holder, a second phase shifting network coupling the anode and cathode of the other of said pair of electron discharge tube systems to a pair of terminals of 

